The invention relates to the field of aircraft ground proximity warning systems and more particularly to systems providing warning of insufficient terrain clearance.
Prior art ground proximity warning systems, such as the terrain clearance system disclosed in Bateman U.S. Pat. No. 3,946,358 and Bateman et al U.S. Pat. No. 3,944,968, both of which are assigned to assignee of this application, utilized both landing gear and landing flap position signals combined with radio altitude signals to generate a warning in the event terrain clearance is less than predetermined limits. These systems would have provided timely warnings for certain controlled flight into terrain incidents where the aircraft was flown into the terrain when the aircraft was not in a landing configuration. However, there are instances where a pilot may extend the landing gear to act as an aerodynamic brake or drag during certain portions of a flight. Lowering the landing gear in the prior art ground proximity warning systems inhibits the terrain warning mode which has the effect of eliminating a terrain warning protection capability under certain circumstances when it in fact might be needed.
In addition, there have been instances in the past when the flight crew have misread the barometric altimeter by 10,000 feet and as a result have unknowingly flown at cruising speeds within a very close proximity to the ground. The prior art terrain clearance warning systems, as represented in U.S. Pat. No. 3,946,358, in many instances will not provide sufficient warning time at these higher cruise speeds. This is due in part to the fact that they do not provide a warning until the aircraft is within 200 feet of the ground unless it also is descending at a barometric descent rate at a rate great enough to raise the warning threshold to 600 feet.
A failure of the gear or flap switch utilized for a ground proximity warning system input can lead to a loss of confidence in the system since such a failure will manifest itself on a landing approach with a continuous pull-up warning. Since the cause of the problem is not immediately obvious, pilots will tend to lose faith in the system.
Another cause of lack of faith in ground proximity warning systems is the fact that heretofore the majority of the warnings have been in the nature of a single imperative command to pull-up. It has been discovered that pilots are reluctant to immediately respond with a maneuver when they are not certain what the circumstances are that have triggered the warning.